1.Field of the Invention
The present invention relates to a method for manufacturing a multilayered substrate, in which a silicon layer is overcoated with diamond, or a diamond layer is overcoated with silicon, for a semiconductor device, as well as a semiconductor device including the multilayered substrate produced by the above-described method.
2. Description of the Related Art
A diamond produced by vapor-phase synthesis on a wafer-shaped substrate made of silicon or the like has a thermal conductivity about 5 times larger than that of copper at room temperature, and is expected to be used as an ultimate heat spreader in a semiconductor device field in which an increase in packaging density and operation frequency have progressed significantly.
In order to take advantage of an excellent thermal conduction property of diamond in a semiconductor device, it is desirable to have a configuration in which a semiconductor layer and a diamond layer are adhered to each other and the thermal conductivity across the adhered interface is high, that is, a multilayered structure composed of the diamond layer and the semiconductor layer, wherein the semiconductor layer is made of various semiconductor materials, for example, silicon, SiGe, silicon carbide, nitride semiconductors, e.g., gallium nitride, gallium arsenide based semiconductors, and group II to group VI semiconductors.
However, it is not easy to produce a diamond/semiconductor multilayered substrate for a semiconductor device. This is because the diamond is usually produced by vapor-phase synthesis at a high temperature of about 800° C. and, thereby, the multilayered substrate is warped and deformed significantly due to a stress resulting from the difference between thermal expansion coefficients of silicon and diamond when the temperature of the multilayered substrate after formation of the diamond layer is returned to room temperature in the case where a semiconductor layer is formed from silicon. A substrate exhibiting such a warp and/or deformation is not suitable for a substrate to be used for a semiconductor device.
Other reasons include that the diamond is an extremely hard material and, therefore, is resistant to being cut when individual elements are separated after a plurality of elements are formed on the multilayered substrate. Furthermore, a surface of diamond produced by vapor-phase synthesis generally has significant roughness and, therefore, the diamond is unsuitable for a substrate to be used for a semiconductor device. Although it is technically possible to flatten the diamond layer surface by polishing, such surface processing is not practical since a long time is required.
Consequently, in order to suppress the warp and deformation of the substrate, a method in which a diamond layer is formed locally in only a specific region on a semiconductor layer, a method in which silicon layers are formed locally on a support and a diamond layer is formed between individual silicon layers (refer to U.S. Pat. No. 5,131,963), and the like have been proposed previously. FIGS. 7A to 7E are sectional views showing the method for manufacturing a multilayered substrate in order of the procedure described in U.S. Pat. No. 5,131,963. The method for manufacturing a multilayered substrate described in U.S. Pat. No. 5,131,963 is a method developed from a method for manufacturing a silicon on insulator (SOI) substrate. As shown in FIG. 7A, a high concentration of B is diffused on one surface of a second silicon substrate 110 to form an etch-stop layer 111.
As shown in FIG. 7B, silicon is epitaxially grown on this etch-stop layer 111 to form a silicon layer 112 and, thereafter, a diamond layer 113 is formed thereon. If necessary, as shown in FIG. 7C, a polycrystalline silicon layer 114 serving as a support layer is further formed on the diamond layer 113. Subsequently, as shown in FIG. 7D, the silicon substrate 110 is removed by wet etching. As shown in FIG. 7E, the etch-stop layer 111 is removed by ion etching or the like, so that a diamond/silicon multilayered substrate 100 is produced.
However, the above-described known technologies have the following problems. In the method for manufacturing the multilayered substrate described in U.S. Pat. No. 5,131,963, since a chemical vapor deposition (CVD) step of the diamond is conducted at a high temperature of about 800° C., there is a problem in that the silicon layer 112 is contaminated by heavy metals, alkali metals, and the like. Therefore, the method is not suitable for a substrate to be used for a semiconductor device.
This contamination problem of silicon layer can be avoided by applying a bonding technology used in a method for manufacturing an SOI substrate. However, in this method as well, there are problems in that it is extremely difficult to control the process for removing the silicon substrate by polishing or dissolution after bonding, and it is difficult to leave a uniform and thin-film shaped silicon layer. The method for manufacturing a diamond/silicon multilayered substrate, the method taking advance of the bonding technology, has also a problem in that an excellent thermal conductivity of diamond cannot be fully exhibited since the thermal conductivity of the interface between the diamond layer and the silicon layer is extremely small.